1. Field of the Invention
This present invention is related in general to methods and structures for filtering radio waves. More particularly, the invention is directed to methods and structures for fabricating lightweight cavity resonator filters.
2. Description of the Prior Art and Related Background Information
Embodiments disclosed herein are related to a family of electrical circuits generally referred to as cavity resonator filters, which are used in radio frequency transceiver chains. Cavity resonator filters aid with receiving and transmitting radio waves in selected frequency bands. Typically, such filter structures are formed by coupling a number of coaxial cavity resonators or dielectrically loaded cavity resonators via capacitors, transformers, or by apertures in walls separating the resonators. It is noticeable that, unlike the general trend in electric and electronic devices where in recent years significant miniaturization has been achieved, efforts to downsize radio frequency (“RF”) filters have been inhibited. This is primarily due to the fact that, to meet low loss and high selectivity requirements, air-cavity filters with dimensions approaching a fraction of free space wavelength are required. U.S. Pat. No. 5,894,250 is an example of such a filter implementation. FIG. 3 depicts a coaxial cavity filter that is commonly realized in practice which can achieve the electrical performance requirements.
The pursuit of improving the RF bandwidth efficiency in cellular infrastructure has led to increasingly stringent filtering requirements at the RF front end. High selectivity and low insertion loss filters are in demand in order to conserve valuable frequency spectrum and enhance system DC to RF conversion efficiency. Filter structures with spurious-free performance are needed to meet the out-of-band requirements. Furthermore, it is also desired that such filters have both low costs and small form factors to fit into compact radio transceivers units, often deployed remotely for coverage optimizations. The size and weight constraints are even more exasperated by the advent of multiple-input multiple-output (“MIMO”) transceivers. Depending on implementation in a MIMO system, the number of duplexer filters may range from two to eight times that of a single-input single-output (“SISO”) unit, all of which requires smaller and lighter filter structures. The desire for smaller size conflicts with the electrical performance requirement that resonators achieve very high unloaded Q-factor, which demands larger resonating elements.
An RF bandpass filter can achieve a higher selectivity by increasing the number of poles, i.e., the number of resonators. However, because the quality factor of the resonators is finite, the passband insertion loss of the filter increases as the number of resonators is increased. Therefore, there is always a trade-off between the selectivity and the passband insertion loss. On the other hand, for specified filter selectivity, certain types of filter characteristics that not only meet the selectivity requirement, but also result in a minimum passband insertion loss, are required. One such filter with these characteristics is the elliptic function response filter. Notable progress has been made on improving the size, and the in-band and out-of-band performance of the filters. However the size and the associated weight reduction of such structures present formidable challenges in remote radio head products.
FIG. 1 depicts the equivalent lumped element circuit schematic of a bandpass filter with capacitive coupling. FIG. 2 shows the distributed implementation where combinations of lumped and distributed components are being used. This filter structure is known as a combline filter. In this structure, the coaxial resonators are formed by a section of transmission line, the electrical length of which is typically between 30° and 90°. The electrical length of distributed lines dictates the position of spurious bandpass response of the filter in its stop band. The employment of the lumped capacitive elements allows for tunability but the mixed lumped distributed structure improves the spurious response suppression. For these reasons, the combline filter structure is very popular in practice. The implementation of the elliptic response is aided by the application of cross-coupling between the resonators.
Most cellular standards operate in Frequency Division Duplex (“FDD”) mode. This means that for each transceiver, there are a pair of filters forming a duplexer filter structure. As mentioned earlier, more recent architectures, such as MIMO systems, incorporate several duplexers packaged in a single radio enclosure. The relatively large-sized cavity resonators coupled with expected large filter selectivity means that the duplexer(s) practically occupies a large space and forms the main mass of a remote radio head (“RRH”) unit. This is an insurmountable design challenge particularly in the sub-gigahertz bands that are allocated to mobile telephony services.
The forgoing discussion defines the mechanical structure of a typical filter. The structure is normally machined or cast out of aluminum. In order to reduce the weight, the excess metal is machined off from the main body of the structure. This arrangement is shown in FIG. 3.
Accordingly, a need exists to reduce the weight of cavity resonator filter structures.